1. Field of the Invention
The present invention relates to a noise reduction apparatus and method for reducing noise in an image. Further, the present invention relates to a program for causing a computer to execute the noise reduction method and a computer-readable recording medium having stored thereon the program.
2. Description of the Related Art
The numbers of pixels and the sensitivities of CCD's, which are used in digital cameras and the like, have become higher. As the sensitivities of the CCD's have become higher, the influence of noise contained in images obtained by imaging has become a problem. Therefore, various kinds of techniques have been proposed to reduce the noise contained in the images. For example, the amount of noise (noise amount) in each pixel of an image and an imaging condition are estimated, and the estimated amount of noise is corrected based on the imaging condition, and noise in the image is reduced based on the corrected amount of noise (please refer to U.S. Patent Application Publication No. 20050099515). Further, in another method, CCD-RAW data that has been output from a CCD is divided into color components of R, G and B, and noise reduction processing is performed while correlation among the color components is maintained (please refer to U.S. Patent Application Publication No. 20060039039).
Further, in the method disclosed in U.S. Patent Application Publication No. 20060039039, the level of the signal value of a target pixel, which is a target of processing in the image, is shifted so that an average value of the signal value of the target pixel and the signal values of pixels in a predetermined area in the vicinity of the target pixel becomes a value at the origin of a color space of each of a plurality of color components. Then, an operation for noise reduction processing is performed on the target pixel, the level of the signal value of which has been shifted, based on a statistic representing the noise amount of the target pixel. Then, the level of the signal value of the target pixel, the noise of which has been reduced, is restored based on the amount of the level that has been shifted.
However, in the method disclosed in U.S. Patent Application Publication No. 20060039039, the statistic representing the noise of the target pixel is calculated based on the signal value of the target pixel and the signal values of the pixels in the predetermined area in the vicinity of the target pixel, and the operation for noise reduction is performed by using the statistic. Therefore, when the sensitivity of imaging becomes higher and the amount of noise increases, the effect of noise reduction may become too strong and edges and fine patterns included in the image may become blurred (unsharp).